Denture Base Polymers PDF

Denture base polymers Dr. Amr Aly/ Mohamed Said PLEASE REMEMBER  Polymers They are materials formed of several large molecules (polymer chain) with high molecular weight. These molecules are formed when smaller molecules named monomers are connected together through covalent...

Denture base polymers Dr. Amr Aly/ Mohamed Said PLEASE REMEMBER  Polymers They are materials formed of several large molecules (polymer chain) with high molecular weight. These molecules are formed when smaller molecules named monomers are connected together through covalent bonding. Monomer—Chemical compound that can react to form a polymer. Polymer—Chemical compound consisting of a large organic molecule (“macromolecule”) formed by the union of many smaller repeating units (mers). Basic nature of polymer The term polymer denotes a molecule that is made up of many (poly) parts (mers). The (mer) ending represents the simplest repeating units from which the polymer is formed. Polymethyl methacrylate is a polymer having chemical structural units derived from methyl methacrylate. Impression materials, Denture base (Acrylics) materials, Soft lining materials, Fissure sealants  Copolymers Polymer molecules may be repeated from a mixture of monomers, and they are called co- polymer. Random copolymer in which the two monomers enter into the copolymer in a relatively random manner along the chain Alternating copolymer --- two monomers enter into copolymer in an alternating manner  Copolymers Block copolymer is a linear copolymer with one or more long uninterrupted sequences of each polymeric species. Graft copolymer is a branched copolymer with a backbone of one monomer to which are attached to one or more side chains of another monomer. Polymer structure The spatial structure of the polymer molecules is important in determining the properties of polymers. There are three basic types of structures: linear, branched, and cross-linked. The linear and branched molecules are separate and discrete, while the cross-linked molecules are a network structure that may result in the polymer becoming one giant molecule The cross-linked polymers flow at higher temperatures in addition; they don't absorb liquids as readily as either the linear or the branched materials. Polymerization process Polymerization—Chemical reaction in which monomers of a low molecular weight are converted into chains of polymers with a high molecular weight. The molecular weight of the polymers has an effect on the physical properties of the polymer. The higher the molecular weight, the higher the softening and melting points and the stiffer the plastics. Polymerization is never entirely completed, but there remains an amount of monomer incompletely cooked called the residual monomer, which is the cause of irritation of the soft tissues of the mouth, and hypersensitivity. Types of polymerization reaction The polymerization reactions fall into two basic types: 1. Addition polymerization Free radical polymerization Ring-opening polymerization 2. Condensation polymerization The main difference between the two types is presence of by-product accompanying the condensation polymerization reaction. Polysulfide rubber impression material Stages of free radical polymerization 1. Initiation The initiator is activated by different methods (chemical, heat or light) and subsequently releases free radicals (unpaired electrons) that will transfer its energy to the monomer. 2. Propagation The chain reaction continues until all the monomer changes to polymer, actually the reaction is never completed. 3. Termination The chain process is terminated either by direct coupling of two radical chain ends or by the exchange of a hydrogen atom from one growing chain to another. Inhibition of polymerization Presence of impurities Oxygen Addition of hydroquinone inhibitor to the monomer retard the reaction Factors associated with polymerization: Evolution of heat (exothermic reaction). Reduction in volume. Residual monomer. Crosslinking It is a chemical reaction that links the polymer chains together forming a three dimensional polymer network. Benefits of cross-linking; Increases strength, hardness & rigidity Decreases water sorption & solubility Thermal behavior of the polymer Thermoplastic polymers Thermosetting polymers They are linear or branched structures They are usually cross linked Reversible Irreversible They soften when heated They do not soften again on upon cooling it will harden in the new reheating to the same temperature form. As reheating occurs, they soften again They are soluble They have higher abrasion resistance in organic solvents More dimensionally stable Impression compound Impression silicone Thermal behavior of the polymer Thermoplastic polymers Thermosetting polymers Impression compound Impression silicone NOW: LET’S START THE TOPIC! References Phillips’ Science of Dental Materials - Saunders; 12th edition. Chapter 19 Denture base The denture base is that part of the denture which rests on the tissues and carries the artificial teeth. Functions Support & retain denture teeth Stress distribution Improve esthetics Requirements of denture bases 1. Strength and durability 2. Satisfactory thermal properties (thermal conductivity & C.T.E) 3. Processing accuracy and dimensional stability 4. Chemical stability 5. Insolubility and low sorption of oral fluids 6. Absence of taste and odor Requirements of denture bases 7. Biocompatible 8. Natural appearance 9. Color stability 10. Adhesion to plastics, metals, and porcelain 11. Ease of fabrication and repair 12. Moderate cost Classification of denture base Metallic According to type of material: Non-Metallic Metallic denture base Cast gold alloys. Cast cobalt-chromium. Cast titanium & titanium alloy. Swaged stainless-steel. Advantage of metallic denture base 1. High mechanical properties. 2. Could not be scratched. 3. Transmit heat. 4. Minimal allergy to the patient. 5. Crazing not occur. 6. Minimal bacterial & fungal colonization Disadvantage of metallic denture base 1. Color not matches gingival tissues 2. Not easy constructed & repaired 3. Not easy to be finished & polished 4. Not easy to reline 5. Heavy in weight 6. Relatively expensive 7. Corrosion may occur Non-metallic denture base 1. Heat-cured PMMA 2. Self-cure PMMA 3. Light cure PMMA Advantage of Non-metallic denture base 1. Color matches gingival tissues. 2. Easy constructed & repaired. 3. Easy to be finished & polished. 4. Easy to reline. 5. Light in weight. 6. Not expensive. Disadvantage of Non-metallic denture base 1. Moderate mechanical properties. 2. Could be scratched. 3. Do not transmit heat. 4. May cause allergy to the patient. 5. Crazing & bleaching may occur. 6. Bacterial & fungal colonization may occur. Acrylic resin It is poly-methyl methacrylate polymer that is polymerized by an addition polymerization reaction (free radical). Pure poly-methyl methacrylate is transparent and it is pigmented with different colors. ❑ Types of acrylic denture base material: 1. Heat cured acrylic resin. 2. Self cured acrylic resin. (Cold curing resin)(Auto polymerizing resin) 3. Light cured acrylic resin. 1. Heat cured acrylic - Most common used type for denture base - Free monomers are 0.2-0.5% compared to 3%- 5% in chemical cured type - Base plastics are commonly supplied in a powder-liquid form  Powder Liquid Granules of Prepolymerized poly-methyl Monomer: methyl methacrylate methacrylate (To reduce polymerization Inhibitor: hydroquinone (prevents shrinkage from 21% to 7%) accidental polymerization of monomer in Initiator: benzoyl peroxide the liquid) Plasticizer: Dibutyl phthalate Cross-linking agent: glycol Pigments give tissue like shades Dimethacrylate Glass fibers (for stiffness) (decrease the solubility and water sorption Acrylic fibers simulate the minute blood vessels and increase the resistance to crazing) Barium salts (for radiopacity)  Denture construction steps I. Flasking procedure II. Wax elimination III. Proportioning and mixing of heat cure acrylics IV. Packing V. Trial closure VI. Curing VII. Deflasking, finishing & Polishing Proportioning and mixing Monomer /Polymer ratio The proportion of polymer to monomer is 3:1 by volume and 2:1 by weight. The powder and liquid are mixed with stainless steel spatula and kept in a sealed glass jar during the initial stages of reaction to avoid the loss of the monomer by evaporation. High P/L will cause incomplete wetting of the powder with liquid leading to granular mix. Low P/L will lead to increase shrinkage and porosity of the denture.  Then it gets polymerized Monomer breaks the bond between polymers and (Curing) incorporate it in the mix It will start only after activation!  The mix pass through the following physical stages: 1. Sandy stage The polymer gradually settles into the monomer and a somewhat fluid incoherent mass is formed which is grainy. 2. Stringy or sticky stage The monomer attaches the surface of the polymer beads. This stage is characterized by a stringiness and adhesiveness if the mixture is touched or pulled apart. 3. Dough stage The monomer diffuses into the polymer gradually and the mass becomes more saturated with polymer in solution, it becomes smooth and dough like. It is no longer tacky and does not adhere to the wall of the mixing jar. 4. Rubbery stages The monomer disappears by evaporation and by further penetration into the polymer. The mass becomes more cohesive and rubber like. 5. Stiff stage Evaporation of free monomer occurs. The mass is very dry & resistant to any deformation. Working time: The time to reach the dough stage Dough time: the amount of time where it remains at dough (should be more than 5 minutes) During this time packing of material in the mold In heat cured resin, polymerization occurs above 70 degrees C. But you have to consider the exothermic reaction of polymerization process, otherwise, BOILING will occur. Even after temperature control, boiling may still occur at THICK sections of acrylic due to poor thermal dissipation in thick sections leading to ????? Curing To transform methyl methacrylate to poly-methyl methacrylate. Curing polymerization cycles: 1. Long cycle ( 9 hours): Flask is placed in water bath at room temperature and then temperature is raised gradually to 65 ᵒC in 30 minutes and then temperature is raised to 80 ᵒC for 8 hours and then kept at boiling for 30 minutes. (for bulky prothesis) 2. Short cycle (2.5 hours): Flask is placed in water bath at room temperature and the temperature is raised gradually to 65 ᵒC in 30 minutes. And then temperature is kept at 75 ᵒC for 90 minutes and then kept at boiling for 30 minutes. IT SHOULD COOL DOWN GRADUALLY TO PREVENT DISTORTIONS  2. Chemically cured acrylic It has the same composition as that of heat-cured acrylic denture base, but some differences exist The liquid contains a chemical activator responsible for starting the polymerization reaction at room temperature as tertiary amine (N,N-dimethyl-p-toluidine) that reacts with the peroxide initiator leading to production of free radicals to initiate the polymerization reaction. -Used for making special trays -In-office repair of denture base 2. Chemically cured acrylic Advantages 1. Denture base fabrication in short time. Disadvantages 1. Color instability (Due to tertiary amine oxidation by time). 2. Higher residual monomer that could irritate the patient’s soft tissues. 3. Lower mechanical properties as stiffness. 4. Higher porosity. 3. Light cured acrylic Composition Resin matrix: UDMA (Urethane dimethacrylat) and PMMA. Fillers: microfine silica fillers Photoinitiator: comphroquinon Activator: visible blue light (400-500 nm) It supplied in sheets that is adapted to the cast then photopolymerized inside a light curing unit. Applications Denture bases, repair of broken dentures and custom trays. Advantages 1. No MMA monomer found → reducing the possibility of allergic reaction. 2. Lower rate of polymerization reaction→ better fitness of denture base. 3. Short processing time with no need for flasking. Disadvantages Lower mechanical and physical properties Properties acrylic denture bases 1. Mechanical properties Conventional heat-accelerated acrylic resins are still the predominant denture base materials in use. These materials are typically low in strength, soft and fairly flexible, brittle on impact, and fairly resistant to fatigue failure. Strengthening of acrylic denture base could be done by incorporation of fibers as carbon, ultra-high modulus polyethylene fibers, and glass fibers. Properties acrylic denture bases 2. Physical properties a. Thermal conductivity Dental plastics are poor thermal and electrical conductors. Low thermal conductivity allows plastic denture bases to serve as an insulator between the oral tissues and hot or cold materials placed in the mouth. b. Coefficient of thermal Expansion Quite high coefficients of thermal expansion. This does not present problem except with porcelain teeth which may gradually loosen and be lost or may lead to crazing of denture base. Properties acrylic denture bases 2. Physical properties c. Water sorption Acrylic resin can absorb water and result is its expansion. After processing, the denture is immersed into water as water sorption results in 0.46% expansion which compensate the thermal shrinkage. During service, the denture should be kept wet all times. Properties acrylic denture bases 3. Biocompatibility Residual monomers may produce an allergic reaction. The allergic reaction tends to be immediate and is more likely to occur with cold- cure resin (more residual monomers). When patients are known to have suffered from an allergic reaction: 1. Processing the denture for extended periods (such as 24 versus 8 hours) may be helpful. 2. Processing in a water bath for 7 hours at 70°C, followed by boiling for 1 hour. 3. Vinyl acrylic or light-activated denture base materials are an alternative. Properties acrylic denture bases 4. Polymerization shrinkage The density of methyl methacrylate monomer is only 0.945 g/cm3, compared with 1.16 to 1.18 g/cm3 for poly- methyl methacrylate. This increase in density is accounted for by an approximate 21% decrease in volume of monomer during polymerization. Denture teeth Denture teeth come in a variety of shapes, sizes, and shades. The shape is chosen to match that of the patient's natural teeth, usually as judged from an old photograph. Another technique is to use the shape of the face to select the tooth shape. The size is determined by the size of the patient's arch. Often, the patient desires white teeth and must be advised to the true color of natural teeth, because bright white teeth will look artificial. Denture teeth requirements They should have good appearance, resembling natural teeth in shape, color and translucency. There should be good attachment between the artificial teeth and the denture base. The artificial tooth and base materials should be compatible. The artificial teeth should be of low density in order that they do not increase the weight of the denture. Denture teeth requirements The artificial teeth should be strong and tough in order to resist fracture. They should be hard enough to resist abrasive forces in the mouth and during cleaning. They should allow grinding with a dental bur so that adjustments to the occlusion can be made by the dentist at the chair side. Denture teeth Acrylic resin materials teeth Porcelain teeth Acrylic resin teeth Most denture teeth are made from acrylic resin much like that used to construct the denture base. Denture teeth have more cross-linking agent added which are resistant to crazing. Because the teeth are constructed under controlled conditions at a manufacture, they are stronger than the acrylic material used for the denture base. Acrylic resin teeth Acrylic denture teeth are “chemically” bonded to the acrylic denture base during processing of the denture. Porcelain teeth Porcelain teeth are made by manufacturers in much the same shapes, sizes, and shades as acrylic teeth. Porcelain teeth are much harder and more stain resistant compared to acrylic teeth. Porcelain teeth are held in the denture by the mechanical undercuts of pins that are embedded in the back of the denture tooth. Porcelain teeth Porcelain teeth are rarely used because: 1. They excessively wear the opposing teeth. 2. They cause trauma and bone loss in the supporting and opposing alveolar ridges. Thank You !!!